Calorimetric analysis of lambda cI repressor binding to DNA operator sites

Area of Science:

• Molecular Biology
• Biophysics
• Thermodynamics

Background:

• Bacteriophage lambda cI repressor regulates gene expression by binding to specific DNA operator sites.
• Understanding the thermodynamics of repressor-DNA interactions is crucial for elucidating gene regulation mechanisms.

Purpose of the Study:

• To thermodynamically characterize the binding of bacteriophage lambda cI repressor to DNA operator sites.
• To investigate the role of specific DNA sequences and repressor dimerization in binding.
• To compare the thermodynamic profiles of cI repressor binding with other DNA-binding proteins.

Main Methods:

• Isothermal titration calorimetry (ITC) to determine binding enthalpies and heat capacities.
• Differential scanning calorimetry (DSC) to assess the impact of DNA binding on repressor thermal stability.
• Site-directed mutagenesis to study repressor dimerization effects.

Main Results:

• cI repressor binding to DNA operators is characterized by a large negative enthalpy and negative heat capacity, leading to enthalpic dominance across physiological temperatures.
• Repressor monomers do not bind to half-sites; binding occurs to full operator sites with dimeric stoichiometry.
• DNA binding reduces the cooperative unit of N-terminal domain unfolding to two monomers, confirming dimer binding.
• Nonadditivity in binding enthalpies and heat capacities was observed for multiple operator site combinations.

Conclusions:

• The enthalpic dominance of cI repressor-DNA binding, contrasting with entropically dominated systems, may stem from its N-terminal "arm" interactions.
• cI repressor binds DNA exclusively as a dimer to full operator sites, highlighting the importance of dimerization and specific DNA sequences.
• These findings provide critical thermodynamic insights into the molecular mechanisms of bacteriophage lambda gene regulation.